Effect of Separate Estimation of Ks Coefficient in Maize Growth Stages on Estimating the Actual Crop Evapotranspiration Rate, under Water Stress Conditions

The correct estimation of crop evapotranspiration stress coefficient (Ks), causes the accurate estimation of evapotranspiration under water stress conditions. In this research, maize was planted in mini-lysimeter and its daily evapotranspiration amount was measured. The experiment was performed in a completely randomized design. Water stress was applied by soil water depletion at levels of 55%(I1), 65%(I2), 75%(I3) and 85%(I4) of total available water (TAW). In control treatment (I0), the soil water amount was kept in readily available limit. The readily available water limit (RAW) was considered as constant in the growing season and its rate was defined as the depletion of 40% of TAW. In another method, the RAW limit was estimated separately at the maize growth stages of initial, development, mid and late. At the time of increasing the stomatal resistance of leaves (due to water stress), soil moisture was measured. The RAW limit in the mentioned growth stages, was estimated to be 45%, 65%, 60% and 70%, respectively. Therefore, the Ks coefficient was estimated in two methods of variable (in four growth stages) and the constant (in the growth period). Maize evapotranspiration amounts under water stress were estimated by multiplying the control evapotranspiration amount on the Ks coefficients. Then, the actual and estimated amounts of maize evapotranspiration (under water stress conditions) were compared. The results showed that using the constant Ks coefficient for estimating the maize evapotranspiration amount in the growth stages, was not successful. By using the variable Ks coefficient, was considered the maize sensitivity to water stress in the growth stages. Therefore, a suitable estimation of maize evapotranspiration amount was performed under water stress conditions. As a result, for determination the maize evapotranspiration amount under low irrigation management conditions, must be considered the separate Ks coefficients in the growth stages.